The present invention relates to an electrostatic copying apparatus for electrostatically transferring an image formed on an image forming body to a copying medium and, more particularly, to an electrostatic copying apparatus with a transfer roller.
Recently, there has been proposed a transfer device with a transfer roller as a means for transferring a toner image formed on a surface of an image forming body such as a photosensitive layer of a photosensitive drum onto a copying medium such as a copying paper.
The transfer device is provided with a transfer roller which is rotatably disposed touchable to the outer peripheral surface of the photosensitive layer. A bias voltage is applied to the outer peripheral surface of the transfer roller. A copying paper being supplied is nipped between the outer periphery surfaces of the photosensitive layer and the transfer roller. At the paper nipping portion, a toner image formed on the photosensitive layer surface is electrostatically attracted from the photosensitive layer surface onto the paper under the applications of the bias voltage. In this way, the toner image is transferred onto the paper. The image transfer by means of the transfer roller is very effective for the electrostatic copying apparatus of the type in which a toner image has been formed temporally on the photosensitive layer before the execution of copying.
In the image transfer using the transfer roller, however, when a part of a toner image for a paper of large size is transferred to a paper of small size, the toner of the toner image portion formed on the area of the photosensitive layer which is other than the area defined by the small size paper directly adheres to the corresponding area on the outer peripheral surface of the transfer roller. In the next transfer stage using a large size paper, the toner left adhering to the outer peripheral surface of the transfer roller adheres to the reverse side of the large size paper to soil the reverse side of the paper.